[feature request] add a sub command for dfdaemon to validate configuration

[inoc603]

Why you need it?

Provide an easy way to answer most 'Why my setup doesn't work' problems and help users find errors in their configuration.

How it could be?

dfdaemon check [-c config] [image]

• check /etc/dragonfly.conf
• parse and check for errors in /etc/dragonfly/dfdaemon.yml or config file specified by -c
• check docker configuration
• check http_proxy for docker daemon
• if image is present, check if the download url will be proxied with dfget

Other related information

[lowzj]

sounds good!